Couplings are known as machine elements for transmitting the output of a prime remover to a driven machine. Among these couplings, there is known a coupling in which the output shaft (hereinafter, referred to as the “taper shaft”) of prime mover, in which the outer periphery of the shaft end portion is formed into a tapered shape, is fitted in a hub, into which the shaft end portion is inserted and which has a hollow portion the inner periphery of which is formed into a tapered shape. This coupling structure is used to transmit a high output, for example, to transmit the output of a steam turbine to a compressor, or to transmit the power of an engine to a propeller shaft.
A taper coupling structure (hereinafter, referred simply to as a coupling structure in some cases) is configured so that when the taper shaft is fitted in or removed from the hub, oil is supplied to a fitting surface between the taper shaft and the hub and thereby the diameter of the hub is expanded (the diameter is increased) to enable the fitting and removing operations. For this purpose, in the coupling structure, an oil path is provided in the taper shaft (or the hub), and an oil groove connected with the oil path is provided in the outer peripheral surface of the taper shaft (or the inner peripheral surface of the hub), whereby oil is supplied to the fitting surface via the oil path and the oil groove.
The fitting and removing operations in the coupling structure can be accomplished not only by using an oil pressure as described above but also by heating the hub to permit heat expansion thereof. In the case where the hub is large, however, the heating work is not easy to do.
Therefore, in the coupling structure, the fitting and removing operations are accomplished mostly by using the oil pressure.
The coupling structure in which the fitting and removing operations are accomplished by using the oil pressure has a problem that oil splashes out of the oil groove and pollutes the surroundings as described in Patent Document 1. Patent Document 1 proposes that the inner peripheral surface of the hub is subjected to microgroove fabrication (the depth: 5/100 mm or smaller). According to Patent Document 1, due to the microgroove fabrication, oil as a perfect fluid is not present on the inner peripheral surface of hub, and resultantly, even if the taper shaft is removed and the oil is exposed to the space, the oil does not splash as a fluid, and remains in a state of sticking to the surface portion.